Semi-conducting polymers are well-known, and have a range of uses in a number of electronic devices, including as charge transporting or electroluminescent materials in light emitting diodes, field effect transistors and photo-voltaic devices.
Semi-conducting polymers are conjugated polymers which comprise delocalized pi-electrons along the backbone of the polymer. This delocalized pi-electron system confers semi-conducting properties on the polymer, but unlike conventional semi-conductors, the amorphous chain morphology of semi-conducting polymers provides an inhomogenous broadening of the energies of the chain segments and leads to hopping type transport. Semi-conducting polymers can be used as electroluminescent polymers. When injected with holes and electrons, this transporting characteristic enables charge combination to occur in the polymer. This in turn gives rise to singlet excitons, the decay of which causes the emission of light.
There are various known ways of producing semi-conducting polymers, examples of which include Suzuki polymerization as described in, for example, WO 00/53656 and Yamamoto polymerization as described in, for example, T. Yamamoto, “Electrically Conducting And Thermally Stable π-Conjugated Poly(arylene)s Prepared by Organometallic Processes”, Progress in Polymer Science 1993, 17, 1153-1205. These polymerization techniques both operate via a “metal insertion” wherein the metal atom of a metal complex catalyst is inserted between an aryl group and a leaving group of a monomer. In the case of Yamamoto polymerization, a nickel complex catalyst is used; in the case of Suzuki polymerization, a palladium complex catalyst is used.
For example, in the synthesis of a linear polymer by Yamamoto polymerization, a monomer having two reactive halogen groups is used. Similarly, according to the method of Suzuki polymerization, at least one reactive group is a boron derivative group such as a boronic acid or boronic ester and the other reactive group is a halogen. Preferred halogens are chlorine, bromine and iodine, most preferably bromine. As alternatives to halides, other leaving groups capable of participating in metal insertion include groups include tosylate, mesylate and triflate.
Consequently, at each terminus of the polymer, as well as at the terminus of any side-chain of the polymer, there will be a monomer unit which has only been polymerized at one end. The opposing end of this monomer, which is by definition either a terminus of the polymer or a terminus of a side chain of the polymer, will comprise a leaving group. These reactive leaving groups may be detrimental to device performance and so it is desirable to replace them using a process known as end-capping, in which the leaving group at the terminus of the polymer or the side chain is replaced with phenyl, as described in U.S. Pat. No. 5,777,070. In particular, by replacing the leaving group with a charge-transporting moiety, as described, for example, in Adv. Mater. 1999, 11(8), 671-675, the efficiency of the semi-conducting polymer may be increased, without affecting the electronic properties of the polymer chain.